Smarty template .tpl have bad syntax highlighting

I have some smarty template .tpl files in a private gitlab instance. The new diff syntax highlighting (which I love in general), and also the regular file view, both highlight these files pretty terribly.

For some reason all tags and attributes have a red background. That's the most annoying aspect of it that makes it hard to live with.

My assumption is that some other file format also uses .tpl, and it's trying to highlight errors.